Understanding Message Size Limits
Applies to: Exchange Server 2010 SP3, Exchange Server 2010 SP2
You can apply message size limits to individual messages that move through the Microsoft Exchange Server 2010 organization. You can restrict the total size of a message or the size of the individual components of a message, such as the message header, the message attachments, and the number of recipients. You can apply limits globally for the whole Exchange 2010 organization, or specifically for a particular connector or user object.
As you plan the message size limits for your Exchange 2010 organization, consider the following questions:
What size limits should I impose on all incoming messages?
What size limits should I impose on all outgoing messages?
Does my Exchange 2010 organization have a mailbox quota?
How do the message size limits that I have chosen relate to the mailbox quota size?
Are there users in my Exchange 2010 organization who must send or receive messages that are larger than the specified allowed size?
Does my Exchange 2010 network topology include other messaging systems or distinctly separate business units that have different message size limits?
This topic provides guidance to help you answer these questions.
Looking for management tasks related to Transport servers? See Managing Transport Servers.
Types of Message Size Limits
Following are the basic categories of the size limits available for individual messages:
Message header size limits These limits apply to the total size of all message header fields that are present in a message. The size of the message body or attachments isn’t considered. Because the header fields are plain text, the size of the header is determined by the number of characters in each header field and by the total number of header fields. Each character of text consumes 1 byte.
Note
Some third-party firewalls or proxy servers apply their own message header size limits. These third-party firewalls or proxy servers may have difficulty processing messages that contain attachment file names that are greater than 50 characters or attachment file names that contain non-US-ASCII characters.
Message size limits These limits apply to the total size of a message, which includes the message header, the message body, and any attachments. Message size limits may be imposed on incoming messages or outgoing messages. For internal message flow, Exchange 2010 uses the custom
X-MS-Exchange-Organization-OriginalSize:message header to record the original message size of the message as it enters the Exchange 2010 organization. Whenever the message is checked against the specified message size limits, the lower value of the current message size or the original message size header is used. The size of the message can change because of content conversion, encoding, and agent processing.Attachment size limits These limits apply to the maximum allowed size of a single attachment within a message. The message may contain many attachments that greatly increase the overall size of the message. However, an attachment size limit applies to the size of an individual attachment only.
Recipient limits These limits apply to the total number of message recipients. When a message is first composed, the recipients exist in the
To:,Cc:, andBcc:header fields. When the message is submitted for delivery, the message recipients are converted intoRCPT TO:entries in the message envelope. A distribution group is counted as a single recipient during message submission.
Scope of Limits
Following are the basic categories for the scope of the limits available for individual messages:
Organizational limits These limits apply to all Exchange 2010 and Exchange 2007 servers that exist in the organization. The specified message limits apply to all Exchange 2010 and Exchange 2007 servers that have the Hub Transport server role installed. On an Edge Transport server, the specified limits apply to the specific server.
Global limits Global limits are used when Exchange Server 2003 servers coexist with your Exchange 2010 deployment. The global limits are stored in a different location in Active Directory than the organizational limits and are primarily used by Exchange Server 2003 servers. In an environment where you have both Exchange 2010 and Exchange Server 2003 servers in the same organization, changes that you make to the organizational limits are automatically copied to the corresponding global limits. In Exchange 2010, you can modify the organizational limits by using the Set-TransportConfig cmdlet in the Exchange Management Shell, or by configuring the Hub Transport server organization configuration properties in the Exchange Management Console.
Connector limits These limits apply to any messages that use the specified Send connector, Receive connector, Delivery Agent connector, or Foreign connector for message delivery. Connectors are defined on Hub Transport servers or Edge Transport servers.
Active Directory site links Hub Transport servers use Active Directory sites and the costs that are assigned to the Active Directory IP site links to determine the least-cost routing path from each Hub Transport server in the organization to every other Hub Transport server in the organization. You can assign specific message size limits to the Active Directory site links in your organization. For example, you may want to apply a lower message size limit to an Active Directory site link that represents a connection to a remote office with a low-bandwidth connection. Any messages that exceed the maximum message size limit on any Active Directory site link included in the least-cost routing path won't be delivered and will generate a delivery status notification (DSN) with a value of 5.3.4. For more information about message routing in Exchange 2010, see Planning to Use Active Directory Sites for Routing Mail.
Routing group connectors A routing group connector is used to send and receive messages between Exchange 2010 Hub Transport servers and Exchange Server 2003 bridgehead servers when the organization is running more than one version of Microsoft Exchange. Any messages that exceed the maximum message size limit on any routing group connector won't be delivered. They will generate a DSN with the value of 5.3.4. For more information about routing group connectors, see Upgrade from Exchange 2003 Transport.
Server limits These limits apply to a specific Hub Transport server or Edge Transport server. You can set the specified message limits independently on each Hub Transport server or Edge Transport server.
If you are using Outlook Web App, the maximum HTTP request size limit setting on the Client Access servers also controls the size of messages that Outlook Web App users can send. For more information, see Configure Maximum Message Size in Outlook Web App.
User limits These limits apply to a specific user object, such as a mailbox, contact, distribution group, or public folder.
Organizational Limits
The following table shows the organizational limits, including information about how to configure the limits in the Exchange Management Shell or the Exchange Management Console (EMC).
Organizational limits
| Size limit | Default value in Exchange 2010 | Shell configuration | EMC configuration |
|---|---|---|---|
Maximum size for messages received |
10 MB |
Cmdlet: Set-TransportConfig Parameter: MaxReceiveSize |
Organization Configuration > Hub Transport > Global Settings > Transport Settings > General tab |
Maximum size for messages sent |
10 MB |
Cmdlet: Set-TransportConfig Parameter: MaxSendSize |
Organization Configuration > Hub Transport > Global Settings > Transport Settings > General tab |
Maximum number of recipients per message Note When a message is first processed by a Hub Transport server, an X-header named |
5000 |
Cmdlet: Set-TransportConfig Parameter: MaxRecipientEnvelopeLimit |
Organization Configuration > Hub Transport > Global Settings > Transport Settings > General tab |
Maximum attachment size in Transport rules that apply to all Hub Transport servers in the organization |
Not configured |
Cmdlets: New-TransportRule, Set-TransportRule Parameter: AttachmentSizeOver |
Organization Configuration > Hub Transport > Transport Rules New Transport Rule wizard or Edit Transport Rule wizard |
Global Limits
The following table shows the global limits, including information about where to configure the limits in Exchange System Manager in Exchange Server 2003.
Global limits
| Size limit | Default value | Exchange System Manager configuration |
|---|---|---|
Maximum size for messages received |
10240 KB (10 MB) |
|
Maximum size for messages sent |
10240 KB (10 MB) |
|
Maximum number of recipients per message |
5000 |
|
Connector Limits
The following table shows the connector limits, including information about how to configure the limits in the Exchange Management Shell or the Exchange Management Console (EMC).
Connector limits
| Size limit | Default value | Shell configuration | EMC configuration |
|---|---|---|---|
Maximum header size through a Receive connector |
64 KB |
Cmdlets: New-ReceiveConnector, Set-ReceiveConnector Parameter: MaxHeaderSize |
N/A |
Maximum message size through a Receive connector |
10 MB |
Cmdlets: New-ReceiveConnector, Set-ReceiveConnector Parameter: MaxMessageSize |
Server Configuration > Hub Transport > Receive Connectors > Receive Connector properties > General tab Edge Transport > Receive Connectors > Receive Connector properties > General tab |
Maximum number of recipients per message through a Receive connector |
200 for the Default Client Receive connector 5,000 for the Default Receive connector on Hub Transport servers 200 for the Default Receive connector on Edge Transport servers Note If the number of recipients is exceeded for an anonymous sender, the message is accepted for the first 200 recipients. Most SMTP messaging servers detect that a recipient limit is in effect. The SMTP messaging server continues to resend the message in groups of 200 recipients until the message is delivered to all recipients. |
Cmdlets: New-ReceiveConnector, Set-ReceiveConnector Parameter: MaxRecipientsPerMessage |
N/A |
Maximum message size through a Send connector |
10 MB |
Cmdlets: New-SendConnector, Set-SendConnector Parameter: MaxMessageSize |
Organization Configuration > Hub Transport > Send Connectors > Send Connector properties > General tab Edge Transport > Send Connectors > Send Connector properties > General tab |
Maximum message size through an Active Directory site link |
Unlimited |
Cmdlet: Set-AdSiteLink Parameter: MaxMessageSize |
N/A |
Maximum message size through a routing group connector |
Unlimited |
Cmdlet: Set-RoutingGroupConnector Parameter: MaxMessageSize |
N/A |
Maximum message size through a delivery agent connector |
Unlimited |
Cmdlets: New-DeliveryAgentConnector, Set-DeliveryAgentConnector Parameter: MaxMessageSize |
N/A |
Maximum message size through a foreign connector |
Unlimited |
Cmdlet: Set-ForeignConnector Parameter: MaxMessageSize |
N/A |
Server Limits
The following table shows the server limits, including information about how to configure the limits in the Exchange Management Shell or the Exchange Management Console (EMC).
Server limits
| Size limit | Default value | Shell configuration | EMC configuration |
|---|---|---|---|
Transport rule on an Edge Transport server that only applies to the specific server |
Not configured |
Cmdlets: New-TransportRule, Set-TransportRule Parameter: AttachmentSizeOver |
Edge Transport > Transport Rules New Transport Rule wizard or Edit Transport Rule wizard |
Maximum header size for messages in the pickup directory |
64 KB |
Cmdlet: Set-TransportServer Parameter: PickupDirectoryMaxHeaderSize |
Not applicable |
Maximum number of recipients per message for messages in the pickup directory |
100 |
Cmdlet: Set-TransportServer Parameter: PickupDirectoryMaxRecipientsPerMessage |
Not applicable |
In addition, you can configure the maximum HTTP request length on your Client Access servers that service Microsoft Office Outlook Web App clients. The value configured for this setting will also affect the message size users can submit. For example, if you set this value lower than other message size limits in your organization, your users will not be able to send large messages using Outlook Web App even though they can send the same message using Outlook.
You can configure this setting by modifying the maxRequestLength parameter in the web.config file on your Client Access servers. By default, this file is located in the <Exchange install directory>\V14\ClientAccess\Owa folder. The default value is 30000 KB.
User Limits
The following table shows message size limits that you can configure at the recipient level, including information about how to configure the limits in the Exchange Management Shell or the Exchange Management Console (EMC).
User limits
| Size Limit | Default value | Shell configuration | EMC configuration |
|---|---|---|---|
Maximum message size that can be sent by this recipient |
Unlimited |
Cmdlets: Set-DistributionGroup Set-DynamicDistributionGroup Set-Mailbox Set-MailContact Set-MailUser Set-MailPublicFolder Parameter: MaxSendSize |
For mailboxes: Recipient Configuration > Mailbox Properties > Mail Flow Settings tab For mail public folders: Public Folder Management Console > Public Folder properties > Mail Flow Settings tab Note This setting isn't configurable using the EMC for other recipient types. |
Maximum message size that can be sent to this recipient |
Unlimited |
Cmdlets: Set-DistributionGroup Set-DynamicDistributionGroup Set-Mailbox Set-MailContact Set-MailUser Set-MailPublicFolder Parameter: MaxReceiveSize |
For all recipient types except Mail Public Folders: Recipient Configuration > Recipient Properties > Mail Flow Settings tab For Mail Public Folders: Public Folder Management Console > Public Folder properties > Mail Flow Settings tab |
Maximum number of recipients per message sent by this recipient |
Unlimited |
Cmdlets: Set-Mailbox, Set-MailUser Parameter: RecipientLimits Cmdlet: Set-MailUser Parameter: MaxRecipientsPerMessage |
N/A |
Order of Precedence for Message Size Limits
You can set different message size limits at different levels in the Exchange organization. As a message is routed through your Transport infrastructure, it may be subjected to several different message size restrictions. You should plan your message size restrictions in a way that makes sure that messages in the transport pipeline are rejected as early as possible if they violate message size limits. Generally speaking, you should set more restrictive limits at the points where messages enter your infrastructure. For example, any message size restrictions on your Edge server Receive connectors that receive messages from the Internet should be less than or equal to the message size restrictions you configure for your internal Exchange organization. It would be a waste of system resources for the Edge Transport server to accept and process a message from the Internet that would be rejected by your Hub Transport servers. Make sure that your organization, server, and connector limits are configured in a way that minimizes any unnecessary processing of messages.
One exception to this approach is the user limits. User level limits take precedence over other message size restrictions. Therefore, you can configure a user to exceed the default message size limits for your organization. For example, you can allow a specific group of user mailboxes to send larger messages than the rest of the organization by configuring custom send and receive limits for those users.
The exceptions for the user limits only apply to message exchanges between authenticated users. If a message is sent to or received by a recipient on the Internet, the organizational limits will be applied. For example, assume that you have an organizational message size restriction of 10 MB, but you have configured the users in your marketing department to send and receive messages up to 50 MB. These users will be able to exchange large messages with each other, but they still won’t be able to receive large messages from Internet users because such messages will be coming from unauthenticated senders.
Messages Exempt from Size Limits
The following list shows the types of messages generated by a Hub Transport server or an Edge Transport server and exempted from all message size limits:
System messages
Agent-generated message
Delivery status notification (DSN) messages
Journal report messages
Quarantined messages
Important
Even though they are generated by the system, non-delivery reports (NDRs) are not exempt from message size limits.
However, these messages are still subject to the organizational value for maximum number of recipients in a message. This value is set by the MaxRecipientEnvelopeLimit parameter that you can configure by using the Set-TransportConfig cmdlet in the Shell.
Differences in Message Size Limits Between Exchange 2003 and Exchange 2010
The primary difference in message size limits between Exchange Server 2003 and Exchange Server 2010 is in the handling of recipient limits. Exchange 2003 treats each member of an expanded distribution list as one recipient. Exchange 2010 treats a distribution group as one recipient. This change was implemented to avoid the partial message delivery scenarios that may occur in Exchange 2003.
Partial message delivery occurs in Exchange 2003 if the number of individual recipients and the recipients that are contained within the distribution list exceeds the specified recipient limit. The total number of message recipients isn't known until after distribution list expansion. Message delivery occurs as the distribution list is expanded until the number of recipients reaches the specified limit. The remaining recipients do not receive the message, but at least the sender receives a non-delivery report (NDR) for each unsuccessful delivery. However, if delivery failure reporting is disabled for the distribution list, the remaining recipients wouldn't receive the message, and the sender would not know who did not receive the message.
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